The present invention relates to an installation for treating of a feed gaseous mixture by selective permeation, of the type adapted to be supplied with a nominal feed flow rate of the gaseous mixture, and comprising at least two subgroups of permeators, each subgroup comprising one or several permeators mounted in parallel, and having at least one inlet, a permeate outlet, and a non-permeate outlet.
It relates moreover to a process for treating a gaseous mixture using such an installation.
Installations for the separation of a gaseous mixture by selective permeation comprise permeators which comprise a receptacle in which is arranged a membrane having selective permeability, separating a non-permeate chamber and a permeate chamber.
The receptacle comprises a feed opening, a non-permeate outlet opening and a permeate outlet opening. Supplementally, certain permeators comprise a fourth opening which permits the introduction of a so-called sweeping gas into the permeate circuit. The presence of this opening is however not pertinent to the present invention.
So as to separate a gaseous mixture, the permeator is fed with a stream of the mixture by opening the supply. There is then produced a pressure difference between the two sides of the membrane.
Given that the membrane has a greater permeability for one constituent of the mixture than for another constituent of the mixture, the permeate is enriched in the more permeable constituent whilst the other constituent remains essentially on the non-permeate side.
The two gas flows are then withdrawn from the respective outlet openings.
The permeability of the membrane for a given constituent of the gaseous mixture depends on different parameters, among others: the temperature of the feed gas flow, the pressure of this gas flow, the pressure of the gas on the permeate side and the difference between these two pressures.
This is why the installations are designed for a predetermined flow rate of feed gas, at a certain pressure and at a certain temperature, and for a certain pressure on the permeate side. At this point of use, the installation separates a certain proportion of constituent to be separated from the mixture, called productivity or recovery, at a certain purity.
Very often, for reasons of production, the separation installations are not used at their predetermined use point, the demand for production gas (either the permeate, or the non-permeate) being below the nominal gas flow rate.
In such cases, if the feed flow rate is simply decreased, corresponding to a decrease in demand, it is noted that the productivity increases whilst the purity of the recovered product decreases.
So as to impart a certain flexibility to installations as to feed flow rate, whilst keeping the productivity and the purity constant, numerous proposals have been made.
In the first instance, one could modify the operative conditions of the membrane. This could be done by acting on the operating pressures on the feed side and/or the permeate side, for example by reduction of the pressure difference between the two sides of the membrane or by reduction of the pressure on the feed side, or else by changing the temperature of the gas (see for example U.S. Pat. Nos. 4,806,132, 4,857,082, 5,840,098).
The modification of the use conditions of the permeators is however very limited if it is desired to maintain acceptable performance.
In the second place, the surface of the permeation used could be decreased.
As to this, it has been proposed to install n(n greater than 2) permeators in parallel which are each supplied with a portion of the feed flow. When the gas demand decreases, the number of permeators is decreased as function of blocking the permeate outlet of a corresponding number of permeators, as described for example in U.S. Pat. No. 4,397,661. Each permeator is then traversed by its nominal flow rate, but the permeation takes place only in a portion of the permeators.
In this installation, the feed flow rate remains unchanged during operation under decreased requirement for production. The productivity is correspondingly reduced.
The invention has for its object to provide an installation for the separation of a gaseous mixture by selective permeation, which permits an operation below the nominal feed flow rate whilst keeping a productivity and a purity similar to those at nominal operation, and which permits rapid return to nominal conditions.
To this end, the invention has for its object an installation for treating a gaseous mixture by selective permeation, of the type mentioned above, in which the non-permeate outlet of a first subgroup is connected to the inlet of a second subgroup, and at least one of the first and second subgroups comprises means for at least substantially selectively closing its permeate outlet.
According to particular embodiments, the invention can comprise one or several of the following characteristics:
the connections of the non-permeate outlet of a subgroup to the inlet of a following subgroup are free from any closure means;
each of the subgroups has a same permeation surface;
each of the subgroups, except one subgroup, particularly the upstream subgroup, comprises means for at least substantially closing the permeate side;
the permeation surface is different from one subgroup to the other;
the subgroups are interconnected such that each subgroup has a permeation surface greater than or equal to that of the following subgroup;
each of the subgroups comprises means for at least substantially closing its permeate outlet;
the closure means are all or nothing valves;
the installation comprises means for adjusting the temperature of the feed fluid and/or means for adjusting the pressure of the feed fluid and/or means for adjusting the pressure of the permeate.
The invention also has for its object a process for treating a gaseous mixture using an installation as defined above, characterized in that said closure means of at least one subgroup are at least substantially closed when the real feed flow becomes less than the nominal feed flow rate by a predetermined quantity.
According to the invention, the process can comprise one or several of the following characteristics:
when the closure means are closed, the ratio of the effective permeation surface to the total permeation surface of the installation is substantially equal to the ratio of the real feed flow rate to the nominal feed flow rate;
when an equalization of the ratio of the permeation surfaces to the ratio of the flow rates by said closing is not possible, opening means are actuated such that the ratio of the permeation surfaces will be as close as possible to the ratio of the permeation surfaces during said equalization, and greater than this value;
when the ratio of the effective permeation surface to the total permeation surface is not equal to the ratio of the real feed flow rate to the nominal feed flow rate, the temperature of the feed fluid and or/the pressure of the feed fluid and or the pressure of the permeate are adjusted such that the permeate has a predetermined flow rate and purity;
all the non-permeate flow of a subgroup is the supply flow of the following subgroup.